I always WONDERED why they used Mercury bearings
Maybe because the tiny group of engineers and specialists who build astronomical instruments are familiar with mercury, not the alternatives.
My division of an aerospace company loves metals, and it likes generous mid-range alloys like 6061 aluminum and precipitation hardened stainless steels (17-4, 15-5, etc.) When we recently needed a stronger, more fatigue-resistant aluminum alloy that led us to pick from the 7000 series, we had to learn partly by trial and error how to deal with its its forging, heat treating, and repair/rework behavior. It's been an expensive learning process figuring out an aerospace industry workhorse alloy because no one on the project has worked with it before.
If it hadn't been for a customer both willing to fund our learning curve and remain unyielding on over-the-top safety margins, we'd be back to 6061 in a heart beat.
So I can completely imagine some instrument maker faced with the tight budget of a grant-funded astronomy lab sticking to old, tried-and-true mercury solutions rather than trying to research and master replacement materials.